The National Highway Traffic Safety Administration has closed its investigation into recent Tesla car fires, as the Palo Alto electric vehicle company announced another voluntary effort to improve safety.

The NHTSA launched the probe following a pair of fires late last year that occurred after Model S sedans ran over roadway debris, but it said in its report that a “defect trend has not been identified.”

It cautioned, however, that “the closing of the investigation does not constitute a finding by NHTSA that a safety-related defect does not exist, and the agency reserves the right to take further action if warranted by new circumstances.”

In a separate but clearly related announcement, Tesla CEO Elon Musk said Friday the company will outfit all new cars and retrofit existing ones for free with a “triple underbody shield” to reduce fire risks.

In a lengthy post on Medium, Musk stressed that the company’s cars are five times less likely to catch fire than conventional gasoline vehicles, but suggested that the company is taking the additional safety measures out of an abundance of caution.

“We felt it was important to bring this risk down to virtually zero to give Model S owners complete peace of mind,” he wrote.

Earlier this year, the company announced a software update that raised the vehicle’s default ground clearance at highway speeds to reduce the chance of severe underbody impacts.

“In this case, Tesla’s revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk,” the NHTSA said.

The three shield layers include a “rounded, hollow aluminum bar” designed to deflect objects or absorb impact; a titanium plate that protects underbody components; and a “shallow angle, solid aluminum extrusion” that absorbs impact, deflects debris and “finally causes the Model S to ramp up and over the object if it is essentially incompressible and immovable.”

Musk added: “During the course of 152 vehicle level tests, the shields prevented any damage that could cause a fire or penetrate the existing quarter inch of ballistic grade aluminum armor plate that already protects the battery pack. We have tried every worst case debris impact we can think of, including hardened steel structures set in the ideal position for a piking event, essentially equivalent to driving a car at highway speed into a steel spear braced on the tarmac.”

Musk has criticized media coverage of the earlier fires as excessive, noting on Friday that the fires late last year “received more national headlines than the other 200,000 gasoline car fires that happened last year in North America alone.”

In a separate media statement, the NHTSA said: “Safety is our top priority. The National Highway Traffic Safety Administration has decided to close our investigation of the Model S Vehicle. NHTSA believes the company’s revision to the vehicle’s ride height and increased underbody protection should reduce both the frequency of underbody strikes and the risk of resulting fires. Consumers should have their vehicles serviced promptly once they receive notification from Tesla Motors.”

The end of the probe removed some uncertainty that had been hovering over Tesla, as a more damning finding could have meant huge expenses for the company. Shares were up more than 3 percent in midday trading.